1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile, for forming an image by utilizing an electrophotographic system, and more particularly, to an image forming apparatus for detecting density of an image formed on a recording material after the image is heated to adjust the density of the image.
2. Related Background Art
As well known, in an image forming apparatus utilizing an electrophotographic system, a visualized image borne on a photosensitive member or a transfer member is transferred to a recording medium such as a piece of plain paper to obtain a recorded image. Thus, the recording medium, to which the visualized image is transferred from the photosensitive member or the transfer member, is conveyed to a fixing apparatus so that the visualized image is fixed thereon by the fixing apparatus, and then the resultant recording medium is discharged.
In recent years, there has been a growing demand for improved high image quality, high stabilizations and the like. With this being the situation, in order to constantly maintain density of an image formed by an image forming apparatus in a proper state, many techniques have been proposed in each of which a reference image having predetermined density is formed to measure its density, an obtained density measured value is compared with a density target value to create a conversion table, and density characteristics of image data are converted using the created conversion table, thereby controlling the density of the image.
As regards a method of measuring density of a reference image required for such density control, there has been known a method in which density of a toner image formed as the reference image is measured before the toner image is transferred to a recording medium, and a method in which density of the toner image which has been transferred to the recording medium but yet to be fixed thereon is measured. However, with those density measuring methods, fluctuations in transfer amount of toner to the recording medium, a degree of fixing of the toner to the recording medium, and the like lead to cause a difference between the density of the actually obtained image and the density of the measured image. As a result, the density could not be controlled with high accuracy.
For this reason, a method has also been proposed in which an image forming apparatus causes its image reading apparatus to read an output image, to thereby control the density based on the readout results. However, according to this method, it has been necessary to set the output image in the image reading apparatus, which has been troublesome. In addition, in the image forming apparatus not including such an image reading apparatus as in a case of a printer, the density could not be adjusted.
Therefore, as for a conventional image forming apparatus, there has been known an apparatus as shown in FIG. 5 in which a density sensor 109 is provided in a recording medium conveying path directly behind a fixing apparatus 108, and density is adjusted based on the density of an image after completion of a fixing process (e. g., refer to Japanese Patent Application Laid-Open No. 2000-132013). Note that, in FIG. 5, reference numeral 101 designates a photosensitive drum, reference numeral 102 designates a laser exposure optical system, reference numeral 104 designates a cleaning device, reference numeral 105 designates a developing apparatus, reference numeral 111 designates a primary charger, and reference numeral 112 designates a transferring charger.
In addition, there has been known an image forming apparatus as shown in FIG. 6 in which a density sensor 209 is provided in a reverse conveying path portion R3 that follows a fixing apparatus 208, to thereby carry out density adjustment (e.g., refer to Japanese Patent Application Laid-Open No. H10-268589). Note that, in FIG. 6, reference symbols 201k, 201y, 201m, and 201c designate photosensitive drums, and reference numeral 203 designates an intermediate transferring member.
Note that in each of the above-mentioned image forming apparatuses, toner contains no wax for enhancing a releasing property from fixing means. However, when it is assumed that the wax is contained in the toner in each of the above-mentioned conventional image forming apparatuses, the following problem arises. That is, since the density sensor is disposed just next to the fixing apparatus, the wax is in a liquid state in the fixed image detected by the density sensor due to the heating during the fixing process. Thus, a progress on coagulation of the wax differs depending on the density degrees of images. For this reason, when the density is measured using a light reflection type density sensor, an amount of reflected light differs depending on the progress on coagulation of the wax, which leads to vary measurement results accordingly. As a result, even when the image density in this state is detected for the measurement, a measured value may vary each time the density is measured. Thus, the density cannot be adjusted with high accuracy.
Accordingly, in a case where a plurality of patches (images) having different density degrees are used as the reference images for the image density adjustment, each of density measured values obtained based on a plurality of patches is compared with a density target value to create a conversion table, and the density characteristics of the image data are converted using the conversion table thus created, thereby controlling the density of the image, it takes long time for a temperature of the wax to decrease to a coagulation temperature (melting temperature) when the image density is high. Hence, the progress on coagulation of the wax differs depending on the patches, and thus a difference between the density measured value and the density target value varies depending on the density of the patches. This becomes a factor by which the accuracy of the density adjustment further gets worse.